1. Field of the Invention
The present invention relates to a film patterning method, an exposure mask and an exposure mask set.
2. Description of the Related Art
These years, continuous demand for fabricating semiconductor devices, such as LSI circuits, in an increasingly microscopic scale has developed into demand for device patterns each with a line width shorter than a wavelength of a light source for exposure which is used in an exposure system available in the commercial market. To satisfy the demand, resolution enhanced technologies including phase shift masks and annular illumination have been adopted for an exposure step.
Mainly, there are halftone masks and Levenson masks in the phase shift masks.
FIG. 1 is a diagram schematically showing an intensity curve I0 of exposure light which has passed through a halftone mask 100. The halftone mask 100 is obtained by forming a mask pattern 102 on a transparent substrate 101 made of quartz or the like. The mask pattern 102 is made of a translucent film, such as a MoSi film, with a light transmittance of approximately 4% to 20%. The film thickness of the mask pattern 102 is set to such a value that exposure light A which has passed through the mask pattern 102 can be shifted in phase by just 180 degrees from exposure light B which has passed through a mask opening 102a. 
In the halftone mask 100, such a phase shift causes the exposure light A and the exposure light B to cancel each other at an edge portion of the mask opening 102a. This makes the intensity curve I0 rise sharp compared with dotted line (it raised from non-phase shift mask), and accordingly makes the contrast of a projection image of the mask opening 102a sharp. This makes it possible to obtain a sufficiently fine resolution.
FIG. 2 is a cross-sectional view of a Levenson mask.
The Levenson mask 110 is made of a transparent substrate 101 and a mask pattern 111. A concave portion 101a is formed in the transparent substrate 101. The mask pattern 111 is formed on this transparent substrate 101, and is made of a light shielding film such as a Cr film.
In the Levenson mask 100, phase difference between the exposure lights A and B become 180 degree, where the exposure light A being passed through the concave portion 101a and the exposure light B being passed through a mask opening 111a formed in the flat surface of the transparent substrate 101. This phase difference causes the exposure lights A and B to cancel each other, so that it is made possible to obtain a fine resolution as in the case of the halftone mask.
A double-pitch, double-exposure method as in Japanese Patent Application Laid-open Publication No. 2002-287324 has also been proposed for the device patterns, which is so highly integrated that the sufficient resolution cannot be obtained by the phase shift masks.
In the double-pitch, double-exposure method, a plurality of holes is classified into two groups. Then, using different masks for the respective groups, all of the holes are formed by carrying out the exposure twice. According to this method, a pitch (center-to-center distance) between each two adjacent hole patterns corresponding to holes in the respective masks is set to be more than twice as large as its original pitch. Therefore, it is made possible to make the distance between each adjacent hole patterns wider, and to accordingly make the depth of focus larger during exposure.